DC/DC converters are used to convert an input DC (direct current) voltage to an output DC voltage. A DC/DC converter may buck or boost the input DC voltage. One type of buck converter is a synchronous buck converter. This type of converter typically has a controller, a driver, a pair of switches, and an LC (inductor-capacitor) filter coupled to the pair of switches. The controller provides a control signal (e. g., a pulse width modulated signal) to the driver, which then drives the pair of switches, e. g., a high side switch and a low side switch. The driver alternately turns each switch on and off, thereby controlling the inductor current and the output voltage of the DC to DC converter.
In general, if the pulse width modulated (PWM) signal is high, the high side switch is on and the low side switch is off. This state of switches will be referred to herein as a “switch on” state. In this state, the inductor is coupled to the input voltage source. Accordingly, the inductor current begins to ramp up, and stores the magnetic energy into the inductor. If the PWM signal is low, the high side switch is off and the low side switch is on. This state of switches will be referred to as a “switch off” state. In this state, the magnetic energy stored in the inductor starts to be released. Accordingly, the inductor current begins to ramp down. The duty cycle of the PWM signal determines the time on for the switch on state and the time off for the switch off state. The duty cycle can be adjusted by monitoring the inductor current via a sense resistor or by comparing the output voltage with a reference voltage level.
FIG. 1A shows an existing DC/DC converter 100 operable for converting an input DC voltage VIN to an output DC voltage VOUT. The illustrated DC/DC converter 100 is a synchronous buck converter, generally including a DC/DC controller 102, a controller 104, a pair of switches (switching circuitry) 106 including a high side switch Q1 and a low side switch Q2, and a low pass filter 108. The low pass filter 108 includes an inductor L and a capacitor C. The low pass filter 108 further includes a sense resistor R1 coupled to the inductor L, operable for providing a feedback voltage level to terminals CSN and CSP of the DC/DC controller 102, which is indicative of the amount of current flowing through the inductor L.
The DC/DC controller 102 has a target input terminal SLEW where the desired output voltage can be set. A capacitor CSLEW charges based on the value of the resistors in the resistor divider R2/R3 and the value of the reference voltage REF. The slew voltage on the capacitor CSLEW increases from zero to a peak value, which is determined by the value of the resistors in the resistor divider R2/R3 and the value of the reference voltage REF. The terminal VFB of the DC/DC controller 102 receives a detection signal indicative of the output voltage VOUT. Based on the peak value of the ramp voltage (e. g., the target output voltage) and the detection signal received by the terminal VFB (indicative of the output voltage VOUT), the DC/DC controller 102 can provide a PWM signal PWM1 and a low side switch enable signal EN to the controller 104. Based on the PWM signal PWM1 and the low side switch enable signal EN, the controller 104 controls the state of the high side switch Q1 and the low side switch Q2.
FIG. 1B is a table 120 illustrating examples of various switch states of the high side switch Q1 and the low side switch Q2 in FIG. 1A for various PWM and EN signals. At block 122, the low side switch enable signal EN is a digital one. If the PWM signal PWM1 is a digital one, the high side switch Q1 is on and the low side switch Q2 is off, and then the switching circuitry 106 is in a switch on state. If the PWM signal PWM1 is a digital zero, the high side switch Q1 is off and the low side switch Q2 is on, and then the switching circuitry 106 is in a switch off state. At block 124, the low side switch enable signal EN is a digital zero. If the PWM signal PWM1 is a digital one, the high side switch Q1 is on and the low side switch Q2 is off, and then the switching circuitry 106 is in a switch on state. If the PWM signal PWM1 is a digital zero, both the high side switch Q1 and the low side switch Q2 are off, and then the switching circuitry 106 is in a switch disabled state (skipping mode).
During operations, the ramp voltage received at the terminal SLEW of the DC/DC controller 102 increases from zero to the peak value. The DC/DC controller 102 adjusts the low side switch enable signal EN and the duty cycle of the pulse-width modulation signal PWM1 according to the peak value (the target output voltage) and the detection signal received at the terminal VFB (indicative of the output voltage VOUT), so as to adjust the output voltage VOUT to the target output voltage set by the peak value.
However, the peak value of the ramp voltage of the DC/DC controller 102 is a constant value determined by the value of the resistors in the resistor divider R2/R3 and the value of the reference voltage REF. The adjusted output voltage VOUT is also constant rather than a variable value, which makes it difficult to meet the application requirement for variable output voltages (e. g., battery charging).